1. Field of the Invention
The present invention relates to an optical pickup servo-control apparatus for use with an optical disc player.
2. Description of the Related Art
Servo control is usually used to control an optical pickup for reading recorded information at a desired reading position for the purpose of achieving good reproduction performance in a player for optical discs, such as compact discs (CD) and digital versatile discs (DVD). An optical disc, however, usually contains defects, such as various kinds of flaws and contamination which occur during, for example, the manufacturing process or the use of the disc. Such defects impede a stable operation of the above-mentioned servo control.
FIG. 1 is a sectional view of an optical disc 1, showing the kinds of major defects which may occur. The optical disc 1 includes a protection layer 3, a reflective layer 4 which reflects the light beam emitted from the optical pickup, and a transparent cover layer 5 made of a transparent material such as a plastic and the like.
Referring to FIG. 1, a defect I is a flaw called an xe2x80x9cinterruptionxe2x80x9d in the reflective layer 4, caused in the manufacturing process of the disc. A defect II is a dirt called a xe2x80x9cblack dotxe2x80x9d on the disc""s surface or the transparent cover layer 5. A defect III is a xe2x80x9cfingerprintxe2x80x9d of human fat on the disc surface 7. A defect IV is a flaw called a xe2x80x9cscratchxe2x80x9d on the disc surface 7.
The existence of such defects on a disc causes xe2x80x9cdropoutxe2x80x9d in read signal (RF signal) generated in the optical pickup when the disc is played, and deteriorates the controllability of servo control including tracking control, focusing control, tilt control and the like. A conventional method, for example, for preventing the adverse influence on servo control caused by such defect is to continue servo control by holding the control value of servo control, or the tracking error value, focus error value, or the like to the value before the defect was detected (hereinafter, simply referred to as xe2x80x9cpre-value holdxe2x80x9d).
The servo control, however, becomes unstable when the dropout continues for a relatively long time or the dropout is of a burst type, because a bias or deviation value indicated by a value in the pre-value-hold method (hereinafter, simply referred to as a xe2x80x9chold valuexe2x80x9d) drifts too far from the actual bias value. For example, the drive signal for the optical pickup drastically fluctuates during the period when a dropout occurs, causing servo control instability as shown in FIG. 2. In addition, as the transient phenomenon arises after termination of the dropout, servo control becomes unstable because the focus error signal is delayed in converging into a normal value, for example, as shown in FIG. 3. Therefore, the performance of servo control may be deteriorated or a gross deterioration in reproduction quality may be caused.
The defect such as a fingerprints or a black dot is usually formed over a plurality of tracks in the radial direction of the disc (i.e., in a certain range of a rotational angle of the disc). Servo control also needs to be stabilized effectively against such defects.
Therefore, it is an object of the present invention to provide a servo control apparatus which has a high capability of dealing with various kinds of defects and of performing stable high-precision servo control.
To achieve the object, according to the present invention, there is provided a servo control apparatus of an optical pickup for reading recorded information from a recording medium to generate a read signal, which comprises an error signal extractor for extracting an error signal from the read signal, the error signal indicating a deviation from a servo target value of the optical pickup; a driver for changing the servo position of the optical pickup; an equalizer for equalizing the error signal to generate a drive signal for driving the driver; an integral value calculator for calculating an integral value of the error signal for each predetermined angular section of rotation of a track on the recording medium; a memory for storing each of the integral values for one rotation of the recording medium; a dropout detector to detect an occurrence of a dropout of the read signal; a variation calculator for calculating a variation by comparing the integral value for an angular section of rotation where the dropout occurred with a stored integral value for the identical angular section of rotation prior to the occurrence of the dropout; and a controller for changing an equalization characteristics of the equalizer in accordance with the variation.